Billionaire Facebook board member Peter Thiel has sold most of his remaining stake in Facebook

The shares were sold as part of a pre-determined trading plan designed to protect company insiders from insider trading accusations.

Billionaire venture capitalist Peter Thiel, Facebook's first institutional investor, has sold three-quarters of his remaining stake in the social network, according to a regulatory filing filed on Tuesday.

The sell was part of a pre-determined trading plan designed to protect company insiders from accusations of insider trading. Thiel, who has been a member of Facebook's board since 2005, now owns 59,913 Class A shares in the company after selling 160,805 shares for about $29 million.

Thiel sold roughly 20 million of his 26 million Facebook shares for $400 million following its stock market listing in 2012. He unloaded roughly $101 million worth of additional shares in May 2016. He initially invested $500,000 in Facebook in 2004.